Optical systems are known for carrying multiplexed optical signal channels via optical fibers and associated optical devices. In some cases such systems are known to employ optical amplifiers to batch-amplify the multiplexed signals being carried by the system, i.e., to amplify simultaneously all of the different signal channels within the wavelength range of the passband, such as the C-band, L-band, S-band, etc. Commercially employed optical amplifiers, such as doped optical fiber amplifiers, typically provide an uneven level of gain across the passband. Erbium-doped fiber amplifiers (EDFAs) operating in the C-band (generally about 1525 nm to 1575 nm), for example, produce a well known two-peaked spectral gain profile. It is known, therefore, to employ gain-flattened amplifiers, such as gain-flattened optical fiber amplifiers or other gain-flattened optical amplifiers.
It is known, for example to employ a dielectric thin-film gain-flattening filter with an optical amplifier, e.g., with an EDFA or other doped fiber amplifier, to construct a gain-flattened amplifier. Thin-film gain-flattening filters have advantageous properties, including low insertion loss, small size, economical design and manufacturing costs, acceptable environmental stability, etc. As a result, the spectrum of optical signals passing through the one or more gain stages of the amplifier and the gain-flattening filter (in some cases referred to here, for convenience, as a GFF) in combination can achieve more uniform amplification, i.e., nearly flat gain. In known optical amplifiers of this type, it is common to employ a pump laser as a source of optical power for amplification, i.e., optical power in suitable pump wavelength(s), alternatively referred to as optical pump energy, to provide optical gain to the optical input signals. The amplification wavelengths are multiplexed with the optical input signals passed to the doped optical fiber of the amplifier at a suitable multiplexer (or “mux”), e.g., a fused fiber mux (or “s-mux”) or the like. Typically, an optical amplifier of this type will have more than one gain stage, e.g., multiple coils of erbium doped optical fiber arranged serially along the optical path through the amplifier. In order to have optical pump energy for each of the gain stages, a corresponding number of pump lasers can be used. Alternatively, it has been known to use the amplification wavelengths from a single pump laser in each of multiple gain stages. Because the amplification wavelengths would not suitably pass through a typical GFF positioned between gain stages, it has been known to employ a fused fiber demux to remove the amplification wavelengths from the signal wavelengths after the first gain stage. Downstream of the GFF the amplification wavelengths are multiplexed back into the optical pathway with the signal wavelengths for the subsequent gain stage.
There is increasing need to provide improved optical amplifiers. Also, there is a substantial need to decrease the cost, weight and/or complexity of optical components, including optical amplifiers, especially those used in telecommunication systems and the like. Accordingly, it is an object of one aspect of the present invention to provide improved optical amplifiers that address one or more of those existing needs. In accordance with certain exemplary embodiments, it is an object to provide optical amplifiers that are more economical to produce, more compact and/or lighter or smaller than known optical amplifiers. It is an object of another aspect of the present invention to provide improved gain-flattening components, including gain-flattening components suitable for use in such optical amplifiers. It is an object of another aspect of the present invention to provide improved filter-coated optical chips, including optical chips suitable for use in such gain-flattening components. It is an object of another aspect of the present invention to provide improved methods of making and using such optical amplifiers, GFF components, and optical chips. Additional objects and advantages of the present invention will be apparent from the following disclosure and from the detailed description of certain exemplary embodiments.